And when a species falls from Life's domain
It never gains a place on Earth again.
We may speculate with some approach to certainty on the general appearance of the earth in those days. There was far more water on the surface of the globe; the land surfaces were small and infrequent. The seas may have been shallower than those which we know, but they were far greater in extent. There must have been far more rain and a very much greater number of violent storms arising from the constant condensation of the waters by the rays of the sun. The sun was probably seen far less often in those days, and there are some geologists who believe the earth to have been perpetually covered with cloud, as the planet Venus is now.
Europe as a continent did not exist. A few islands showed their heads above the waves where Germany and Switzerland, Eastern France and Spain now stand. Scotland's rocky islets were probably visible, on the extreme west, though these islands were destined to sink again below the waves. Thence the ocean stretched without a break, as at present, to Canada. A great part of Canada's bleak lands was above the waters; but the United States, except for a few great islands, were submerged. In the southern hemisphere South America, split into numerous long reefs and islands, gave promise of the continent to be; and there were great stretches of land over Brazil, extending to the west where the great chains of mountains now rise. Asia was largely covered with shallow waters, and the whole extent of the northern plains of Africa was sea. So far as we are able to judge the distinctions of climates were less marked then than now, and the conditions seem to have been much more uniform over all the northern hemisphere. This equality of climate lasted into the next or Ordovician period.
The Ordovician period glides insensibly into the Cambrian. There was no distinct break in the succession of life. The species seem to have slowly extended and developed from one of these great periods into another. But the life of the Ordovician era, which has been preserved for us, is much more abundant. Land was beginning to emerge from the sea in greater bulk; life was springing up on the land and was emerging from the sea, perhaps to take up its habitation there. The first insect life appears in the Ordovician. It is not an imposing relic except when seen through the eye of imagination. It is just an obscure wing of an insect which was found impressed on some shales found in the upper Ordovician rocks of Sweden, and all we can say of it is that it belonged to the same class of insects as lady-birds. The existence of this insect shows that there must have been land vegetation and an atmosphere which was suited to active air-breathing things. The other appearance of great interest in the Ordovician rocks is that of the first fish. They were found in Colorado, but they are very much shattered and tell us very little about the animals they represent. These fish were covered with plates, and were evidently thus defended against attack, so that we may surmise the existence of some other animal that preyed on fishes. Whether these fishes were themselves ferocious we cannot, however, say. But that which was the chief characteristic of the Ordovician era was the climax of the trilobite. More than half of all the known trilobites were present in Ordovician times. Only a few of these came over from the Cambrian, while the others make their first appearance in this period. In the next period (Silurian) their numbers fell to one half, and in later periods declined still further, till they disappeared altogether at the close of the Palæozoic era. Some of these curious animals appear to have been able to move very quickly; others would roll themselves up like hedgehogs to defend themselves against attack; and some of the larger ones were from eighteen inches to two feet in length. Next to these in interest were the cephalopod types, marine animals, that may have resembled the swimming nautilus of to-day in some of their developments. They attained to enormous sizes, some of the shells being twelve to fifteen feet in length and a foot in greatest diameter. From this maximum they ranged down to forms smaller than a pipe-stem. Their habits are to be gathered only from their structure and from the habits of their relations in the present seas. Perhaps they floated, shell uppermost, or crawled upon the bottom and preyed on a variety of the weaker forms of life. There appear to have been fewer worms, perhaps because the muddy and chalky sea bottoms of the Ordovician period were less congenial to them than the Cambrian sands.
The changes in the structure of the earth's crust which brought the Ordovician period to an end marked also the beginning of the Silurian period. These changes affected sometimes small areas and were very intense; sometimes they affected larger areas more slightly. It must not be assumed, however, that these changes were necessarily sudden or violent. In examining the rocks now, we see merely the effects, and of these effects it is the more remarkable alone which have survived the march of ages. There was more water on the earth's surface then than now; and side by side with continuous storms of tropical violence, it is extremely likely that volcanoes and earthquake movements were more frequent and more considerable in their effects. The tide of movement by material things may have been faster; and certain it is that the land was now lifting itself up above the shallow seas. Mountains were being built along the coast-lines; behind the coast-lines the continents were shouldering their way upwards in large land areas. North America began to show in this period the first signs of becoming a continent; Europe's countries, or some of them, assumed a distinct existence. With the advent of mountains came streams and rivers; and the streams, fed by the abundant rainfalls, rushed down to the seas in torrents that performed the work of erosion with a rapidity perhaps unequalled by even the greatest rivers of our present-day knowledge—though at first the land areas were not large enough to give rise to streams as long as great rivers like the Amazon or Mississippi. We cannot say exactly what the areas and localities of the water and the land were; but it is safe to assume that at the beginning of the Silurian period beds of sediment brought down by the rivers and the rain were accumulating about the borders of the land, and as far out as the waves and currents were able to convey the earth materials. The climate was still equable and was much the same over great areas of the world's surface, for the forests of warm temperate latitude are, in part, the same as those in Arctic regions. Certain parts of the land appear to have been desert.
Life began to change a great deal in Silurian times. The extensive withdrawal of the sea from great stretches of submerged surface reduced the area of shallow water available for the forms of life that had so richly peopled it during the Ordovician period. Then there came an age during which the sea invaded some of the regions of the earth's crust, and again withdrew, leaving behind it great stretches of water which gradually grew more intensely salt. All these things had naturally a great effect on the development of the plants and animals of Silurian times. We cannot in a brief summary of this kind do more than indicate some of the more conspicuous features. Corals began to spread through the clearer seas: and reef building on a great scale took place, generally some distance from the shores of the land. Other life in great abundance and variety gathered upon or about these reefs, and they became rich depositories of the animals of their day. The Crinoids, which, though animals, are sometimes called the lilies of the sea, developed strongly; sea urchins appeared, and forms akin to barnacles. The ancestors of the pearl oyster and the mussel date from Silurian times; and so do the first Ammonites, those creatures known to the youngest collectors of fossils, and deriving their names from the Canaanitish god Ammon, which had a ram's head. Sea scorpions, sand fleas, king crabs, sea squirts, and worms and fishes of various kinds haunted the Silurian seas. The Silurian fish were most of them armed for defence, some with plates of bone; some of them had their tails stiffly joined to their backbones; some had skin like a prickly pear; some were not unlike the modern shark. The plants have left us many records—liverworts, ferns, and club-like mosses. The growing vegetation gave a new impulse to insect life—plant-lice and cockchafers and the scorpions we have named: and the vastness of their numbers is shown by the fact that they have outlasted the changes and vicissitudes of a myriad generations.
We may conclude this chapter by saying what we imagine of the general appearance of our own islands to have been. At the close of the Silurian period Britain was probably an archipelago, ranging over ten degrees of latitude, like many of the island groups now found in the great Pacific Ocean; the old gneissic hills of the western coast of Scotland, culminating in the granite range of Ben Nevis, and stretching to the Southern Grampians, forming the nucleus of one island group; the South Highlands of Scotland, ranging from the Lammermuir Hills, another; the Pennine chain and the Malvern Hills, the third and most easterly group; the Shropshire and Welsh mountains, a fourth; and Devon and Cornwall stretching far to the south and west. Every spot of the island lying now at a lower elevation than 800 feet above the sea was under water at the close of the Silurian period, except in those instances where depression by subsidence has since occurred.